This invention relates to a method for producing metal carbide grade powders wherein the composition of the wax can be varied to control the linear shrinkage in sintered articles made from the powder.
Metal carbide grade powders are used in making sintered parts such as cutting tools, mining tools and wear parts. It is extremely important that the shrinkage be controlled during sintering to maintain the proper shape and size of the article.
In making carbide grade powders, there is normally a water milling step in which the powder is intimately mixed with a wax binder. In water milling, the binder metal, especially cobalt, is prone to oxidation and most metallic carbides will decarburize in water forming a methane-acetylene gas. These reactions produce powders with unbalanced chemistries. During subsequent sintering of the formed article, the loss of the oxygen volume in particular causes an increase in shrinkage. There is also a tendency of the wax to separate from the powder causing the grinding of the carbide crystals. This results in generation of excessive quantities of "fines" which are associated with increased shrinkage. Up to this time shrinkage was controlled by adjusting the conditions under which the powder was milled with the wax binder, such as length of time of milling, etc. However, varying milling conditions is not always advantageous because there is a possibility that other properties in the subsequently formed sintered and densified article, such as porosity, hardness, and microstructure, would be adversely affected.
Therefore it has become desirable to have a method of making carbide grade powders in which the shrinkage could be controlled without adversely affecting other properties in articles made from the powders, especially without having to vary milling conditions once the proper milling conditions have been set.